This invention applies to isolated switching-mode power supplies and, more particularly, to a method and apparatus for transferring a feedback signal across an input/output isolation barrier in such a power supply.
Switching-mode power supplies are used to convert a varying input voltage into a constant voltage or current. Both the input and output can be alternative current (AC) or direct current (DC). Most of these power supplies require some form of electrical isolation between their input and output either due to a safety regulation or a system requirement. FIG. 1 shows an exemplary isolated switching-mode DC/DC power supply 10 comprising an isolated power stage 12, pulse width modulation (PWM) controller 14, isolated feedback circuit 16 and error amplifier 18.
Different power conversion topologies such as forward, flyback, bridge, etc. can be used in the isolated power stage 12 to adjust its output voltage by controlling the duty cycle of switching. The error amplifier 18 compares the value of the +V.sub.o output voltage to a reference voltage V.sub.ref (represented by a zener diode) using a conventional differential amplifier 18a so as to detect the difference between the output voltage and the pre-set or reference value, and generate an error signal. This error signal is then transferred to the PWM controller 14 through the isolated feedback circuit 16. The PWM controller 14 produces a switching signal with a proper duty cycle based on the received error signal. The switching signal is used in the isolated power stage 12 to control power conversion and to regulate the output voltage V.sub.o.
Optocouplers have been widely used in the isolated feedback circuit 16. They are simple to use and very low cost. However, they have the disadvantages that the current transfer ratio (CTR) of optocouplers has a large tolerance value from part to part, the CTR also changes significantly with temperature and time, and the frequency response of optocouplers is very poor. These shortcomings prevent optocouplers from being used in high performance and high reliability power supplies.
To overcome the shortcomings of optocouplers, different kinds of magnetic feedback schemes have been developed. In U.S. Pat. No. 4,323,961, there is disclosed a magnetic feedback circuit that transfers a digital signal from an output side to an input side of a power supply. In this patent, a PWM controller located in the output side of the power supply generates a digital signal with a proper duty cycle depending on the load demand or the input voltage. The duty cycle of this digital signal is then used to turn on and off a main switch. To be able to operate properly, the circuit requires a steady switching waveform appearing at the output side of the power supply. Any interruption of the switching waveform during no load condition or output short circuit will degrade the performance of this circuit.
Another form of isolated feedback is commercially available from Unitrode Corporation as an isolated feedback generator integrated circuit (UC3901). This device applies an amplitude modulation to an analog error signal generated by an error amplifier in the output side of the power supply. This modulated signal is transferred to the input side through a magnetic isolator. The feedback signal is then de-modulated and fed to a PWM controller. This scheme is very complicated and the frequency response is poor due to the delay time in the de-modulation process.